1. Field of the Invention
The present invention relates to superconducting joints for electrically connecting superconducting wires and, in particular, to a superconducting joint having high quench tolerance and method for manufacturing the same.
2. Description of Related Art
Techniques for electrically connecting superconducting wires are important for practical use of superconducting wires. One of the simplest methods is to solder sufficiently long joint (overlapping) portions of a pair of superconducting wires to each other (hereinafter “soldering method”). Typically, superconducting wires are formed by embedding plural superconducting filaments in a stabilizer (such as copper and aluminum). Therefore, in superconducting joints formed by soldering methods, an operating current flows from one bundle of superconducting filaments to the other bundle of superconducting filaments via a stabilizer, a solder and the other stabilizer. Thus, the joint resistances of superconducting joints formed by soldering methods are relatively large due to the electrical resistances of the solder and stabilizers. For example, even a joint having a joint (overlapping) length of as long as several meters has a joint resistance of as high as 10−9Ω.
Typical apparatuses including a superconducting magnet wound from a superconducting wire are nuclear magnetic resonance (NMR) apparatuses and magnetic resonance imaging (MRI) apparatuses, which are widely used in the biotechnology or medical field. In general, these apparatuses are operated in a persistent current mode, and require a magnetic field decay rate of as extremely low as 0.1 ppm/h or less. Joints used in such apparatuses need to have a low joint resistance of at least less than 10−12Ω. In order to achieve such a low joint resistance, superconducting joints without any intervening non-superconducting resistive conductors are necessary.
Crimp-type joints have been proposed as such a resistive conductor-free superconducting joint. Methods for fabricating a crimp-type joint include: removing the stabilizer and exposing the superconducting filaments at an end joint portion of each of superconducting wires to be connected; inserting, in a metal pipe, the two bundles of the exposed superconducting filaments; and crimping the two bundles together by compressing and crushing the metal pipe. See, for example, JP-A Hei 4 (1992)-319280. In crimp-type joints, a pair of bundles of bare superconducting filaments are in direct contact with each other without any intervening non-superconducting resistive conductors (such as a stabilizer and a solder). As a result, joint resistances of less than 10−13Ω can be achieved. Because of this advantage, crimp-type joints are currently widely used for superconducting magnets in NMR or MRI apparatuses requiring very low resistance joints.
JP-A 2003-022719 discloses a crimp-type superconducting joint in which a powdered superconductor containing magnesium diboride (MgB2) is interposed between a pair of bundles of bare superconducting filaments. The JP-A 2003-022719 describes that the invented crimp-type superconducting joint has a low joint resistance even under a relatively high magnetic field.
Superconducting wires and magnets have a problem called quenching. Quenching is a phenomenon by which the superconductivity of a superconductor suddenly disappears even when the operating current of the superconductor is less than the rated current. Quenching degrades the operational stability and reliability of superconducting apparatuses, and is therefore one of the most serious problems to be solved.
Conventional crimp-type superconducting joints such as the one disclosed in the JP-A Hei 4 (1992)-319280 are more vulnerable to quenching than other superconducting components. This is partly because in a crimp-type superconducting joint, the stabilizer at an end joint portion of each superconducting wire is removed. Stabilizers have a function to suppress temperature rise due to thermal disturbance since they have a high heat capacity, and also have a function to dissipate generated heat. Therefore, crimp-type superconducting joints have poor quench tolerance because of the absence of stabilizer. Also, the quench tolerance of the superconducting joint disclosed in the JP-A 2003-022719 is not sufficiently improved.